1. Field of the Invention
This invention relates to RF probes for nuclear magnetic resonance (NMR) spectroscopy and microscopy and, more particularly, to resonant coils for the transmission and reception of NMR signals.
2. Description of Related Art
In an NMR spectrometer probe, a sample is placed in a static magnetic field which causes atomic nuclei within the sample to align in the direction of the field. Transmit and receive coils, which may be combined in a single coil or set of coils, are placed in the probe positioned close to the sample. The transmit coils apply an RF magnetic field orthogonal to the direction of the static magnetic field, perturbing the alignment of the nuclei. The transmit signal is then turned off, and the resonant RF signal of the sample is detected by the receiver coil. The sensitivity of the spectrometer depends on a number of factors, including the strength of the static field, the closeness of the coupling between the RF coils and the sample, and the resistance of the RF coil.
Currently, most commercial NMR spectrometers use RF coils made of a normal metal, such as copper, or a combination of normal metals. Much research has been devoted to the design of coils for maximum sensitivity. For example, to achieve close coupling, coils have been made that include configurations such as solenoids, saddle coils and birdcage coils, all of which have high filling factors. In each case, however, the resistance of these coil materials has limited their sensitivity. Cooling of RF coils to reduce their resistance has been suggested. However, even when cooled, the sensitivity of conventional normal-metal coils is still limited by their resistance.
The use of superconductors in place of conventional normal metal for RF coils in NMR spectrometers has previously been suggested. For example, U.S. Pat. No. 5,247,256 to Marek describes several RF receiver coil arrangements for NMR spectrometers using thin-film superconducting coils.
The advantage to be obtained with high temperature superconductor (“HTS”) coils is significant. HTS coils have very low resistance and are operable in high magnetic fields at temperatures achievable with currently available refrigeration systems (above 20K). The quality factor, Q, of the coil is a useful measure of the coil's efficiency. Q=ωL/R, where ω/2π is the resonant frequency, L is the inductance and R is the resistance of the coil. Well-designed room temperature NMR coils achieve matched Qs of about 250. Because of the extremely low resistance of HTS coils, coils with matched Qs of 10,000 or more are possible. However, this advantage can only be realized if the other factors necessary for a superior NMR probe are met, such as a reasonable filling factor and high RF and DC field homogeneity.
In addition to Marek, others have reported thin-film superconductor RF coils for magnetic resonance applications. For example, U.S. Pat. No. 5,276,398 to Withers, et al. describes a thin-film HTS probe for magnetic resonance imaging. It discloses a thin-film coil having inductors in a spiral of greater than one turn and capacitive elements extending from the inductors. Withers thus provides a thin film distributed capacitance probe coil. However, magnetic field disturbances by the coil can be a problem, and the current carrying capacity of the coil is somewhat limited.
U.S. Pat. No. 5,258,710 to Black also describes HTS thin-film receiver coils for NMR microscopy. Black discloses several embodiments, including split ring, solenoidal, saddle coils, birdcage coils and coils described as “Helmholtz pairs.” Black's embodiments are essentially conventional NMR coil designs and do not address the unique characteristics of high-temperature superconductor materials. Superconductors are very attractive for use in these coils. They have very low resistance at radio frequencies and, hence, produce little noise. Even so, to obtain high signal-to-noise ratio (SNR), the coils must be as close as possible to the sample. Unfortunately, this means that any magnetization of the coil material will affect the uniformity of the DC polarizing field (B0) over the sample volume, producing a distortion of the spectral line shape and degradation of SNR. Because superconductors are strongly diamagnetic, line-shape distortions could be severe.
Thin-film HTS coils offer design and processing challenges not present with normal-metal coils. First, high-temperature superconductors are perovskite ceramics, which require a well-oriented crystal structure for optimum performance. Such orientation is extremely difficult to achieve on a nonplanar substrate. Generally, such coils are preferably deposited epitaxially on a planar substrate. This makes the achievement of a high filling factor more challenging. It is also desirable for the coil to be deposited in a single layer of superconducting film, without crossovers. Second, the coil must be able to handle relatively high currents while producing a uniform magnetic field and avoiding distortion of the B0 field of the magnet. Even when HTS films are deposited epitaxially on a planar substrate, crystalline defects inevitably occur. This can lead to burn out of fine features of a coil exposed to high currents. Third, it is well known in the art that forming ohmic contacts between an HTS and a normal metal is difficult and generally leads to parasitic losses at the point of contact. To the extent that a normal metal is used in the coil, resistive losses in the metal elements would lessen the advantages gained from the use of the HTS. Thus, an ideal probe should avoid normal-metal conductors in series with the HTS.
U.S. Pat. No. 5,565,778 to Brey, et al. discloses a number of different configurations of a probe for NMR spectroscopy. Each of these configurations uses a coil having conductors mounted on a planar substrate. The conductors are arranged such that the coil includes at least one interdigital capacitor. That is, interleaved conductors having a constant spacing between them are located on the substrate. Each conductor surrounds a central sample location and lies closely adjacent to at least one other conductor. None of the conductors completely surrounds the sample location on its own, but the conductors are in an alternating arrangement such that adjacent conductors have respective breaks in their conductive paths at different radial positions relative to the sample location. This results in a capacitive configuration that forms a coil surrounding the sample location.